A critical design parameter for any linear transmitter is spectral spreading. Nonlinearities in the transmitter produce spectral regrowth in adjacent and alternate channels, or off-channels, that can reduce overall system performance in a multi-user system. Communication system specifications typically place limits on the amount of regrowth that will be allowable in order to maintain good system performance, and each transmitter must keep its spectral regrowth below those limits for all performance conditions, including temperature fluctuations, load variations, and voltage variations.
In the United States Time Division Multiple Access (US TDMA) wireless communication system, transmitter linearity is specified in terms of adjacent and alternate channel leakage power. As defined in Interim Specification 54 (IS-54) and Interim Specification 136 (IS-136) documents, the measurement of off-channel leakage power is performed by determining the total power that a receiver would intercept if tuned to an adjacent or alternate transmit channel, then comparing those measurements to the power that would be intercepted by that same receiver when tuned to the desired transmit channel. The test receiver is defined to have the same square root raised cosine (SRRC) selectivity filter as real receivers in the system. For US TDMA, limits for off-channel leakage power measurements are: (1) adjacent channel power shall be 26 dB down from on-channel power; and (2) alternate channel power shall be 45 dB down from on-channel power.
To maintain linearity in the face of variable conditions, the current design approach is to provide sufficient linearity margin under nominal conditions to allow for performance condition variations. To provide this design margin, the transmitter must be sufficiently far from saturation so that it will not saturate under any of the extremes of its operation conditions. Because proximity to the saturated condition is required for good efficiency, this built-in design margin creates a direct trade-off in designing for good efficiency versus designing for good linearity. A conventional transmitter designed with sufficient linearity at its operating extremes will have degraded efficiency under nominal conditions, while a transmitter with better nominal efficiency will not maintain linearity at its operating extremes. Thus, there is a need for a linear transmitter that is capable of maintaining good efficiency in a variety of operating conditions.